1. Field of the Invention
The present invention relates to white noise suppressing circuit for suppressing white noise of a voice received by a speaker telephone apparatus.
2. Description of the Related Art
When receiving signals arriving via a communication line are passed through a speaker telephone apparatus, noise caused by the circuit elements in the speaker unit is added to the signal. Such noise is amplified by the speaker amplifier and is noticed as a hiss when there is no receiving signal. The receiving signal has the frequency spectrum shown in FIG. 1, and is transmitted over a bandwidth ranging between 0.3 KHz and 3.4 KHz. However, the noise generated in the speaker unit is homogeneous in terms of frequency and is therefore close to white noise.
When there is a voice input, the voice level is far higher than that of white noise and white noise can thus be ignored. However, when there is no voice input, only white noise is amplified by the speaker amplifier and is output. It is therefore heard as undesired noise. Such a problem involving white noise has conventionally been overcome by the use of a circuit which employs a low-pass filter. That is, a receiving signal line is constructed of, for example, a receiving voice input terminal 1, a receiving amplifier 2, a hand-free (HF) circuit 3, a low-pass filter 4, a speaker amplifier 5 and a receiving voice output terminal 6, as shown in FIG. 2, so that all the frequencies above a certain cutoff point are attenuated by the low-pass filter 4. Alternatively, a switch 11 is provided in the speaker amplifier 5, as shown in FIG. 3. When there is no voice input, the switch 11 is turned off to stop the operation of the speaker amplifier 5.
However, when the voice is passed through the structure shown in FIG. 2 in which the high frequency components of the voice are attenuated by the low-pass filter 4 inserted in the receiving signal line, it is muffled and conversations or the like are thus made obscure. In that case, adjustment of the cutoff frequency of the low-pass filter 4 does not work, because increasing clearness of the voice is incompatible with suppression of white noise. Also, in the structure shown in FIG. 3 in which the speaker amplifier 5 is turned off when there is no voice input, the difference in the sound level between when there is a voice input and when there is no voice input is great, and the user may have an unnatural feeling.